Struts 2 required validator

I have read in the struts documentation about 2 validators required and requiredstring.

The required validator checks whether a field value is null.

The requiredstring validator checks whether a field value is null and also whether the field value is empty.

So the required validator will not find any error if a blank value is passed as the field value but requiredstring validator will not allow blank value to be submitted.

I have the following doubt: If the required validator is anyway going to allow blank values to be submitted what is the use of this validator? Suppose I have a field named userName where the required validation is carried out.